In the process of fabricating microelectronic devices such as semiconductor integrated circuits, certain steps typically must be limited to well-defined regions of the semiconductor wafer. These regions must be precisely delimited and the other areas of the wafer must be protected from the action of a particular fabrication step.
Photolithography is a common method of selectively protecting areas of a wafer. According to this approach, a coating of photosensitive photoresist is first layered on the wafer. One method of applying the photoresist coating is spin processing. A spin coater is used for this purpose. The spin coater places a suitable quantity, typically 1 ml to 4 ml, of photoresist in the center of a wafer spinning at an initial rotational speed, e.g., 1000 rpm to 1500 rpm. At some point after the photoresist is placed on the wafer, the rotational speed of the wafer is ramped up to a final spin speed. Typically, the wafer is spun by a chuck to which the wafer is attached by vacuum suction or the like. Centrifugal force causes the photoresist to spread out over the surface of the spinning wafer.
After the photoresist coating is layered on the wafer, it is exposed to light through a photomask. The photomask is formed of a thin metal film or the like having a desired pattern that selectively allows light to pass through the mask and expose the photoresist coating. If the photoresist is of a so-called negative type, the exposed photoresist coating is developed by dissolving and washing away the unexposed regions. If the photoresist is of a so-called positive type, the exposed photoresist coating is developed by dissolving and washing away the exposed regions. In both types of photolithography, the remaining photoresist forms the protective mask on the wafer.
In photolithography processing, it is important to maintain a uniform photoresist coating thickness. To form high-quality semiconductors, not only should the photoresist coating be uniform throughout the entire surface of a single wafer (referred to herein as “intra-wafer uniformity”), the photoresist coating also should be uniform from wafer to wafer (referred to herein as “wafer-to-wafer uniformity” or simply “uniformity”). It is known that the thickness of a photoresist coating depends on many factors including viscosity and temperature of the photoresist solution, spin speed, bake temperature, bake time, bake pressure, process timing delays, spin acceleration, air velocity, humidity, temperature, and pump parameters.
Previous efforts at increasing coating thickness uniformity focused on the temperature and humidity factors because these factors have a significant impact on coating thickness and can be directly controlled in a relatively inexpensive, straightforward manner. For example, to directly control the effects of temperature and humidity, processing can be carried out in an environmental chamber in which the temperature and humidity are maintained at pre-selected, constant levels so that the uniformity variations due to temperature and humidity are avoided.
In addition to directly controlling the temperature and humidity, another approach is to measure the temperature or humidity and then control a different process parameter such as the spin speed, the temperature of the photoresist, and/or the temperature of the wafer in order to achieve uniformity and compensate for temperature and humidity variations. Such an indirect approach is shown in U.S. Pat. No. 5,127,362 (1992) issued to Iwatsu et al. The approach described in Iwatsu involves measuring the temperature and/or the humidity, and then adjusting the spin speed, the temperature of the photoresist, and/or the temperature of the wafer in response to the measured temperature and/or humidity.
The problem with controlling, or compensating for, temperature and/or humidity is that there is an inherent limit to the level of uniformity that is attainable by relying on only these two inputs. As tolerances for circuits manufactured upon semiconductor wafers become more demanding, other factors must be addressed to achieve greater levels of photoresist thickness uniformity.